Hitherto, it has only been possible to apply the process of defibering chips with a low energy input subsequent to preheating the chips under high pressure and high temperature (150.degree.-170.degree. C.), the so-called Asplund process, within the board manufacturing industry, since the pulp resulting from this process is dark in colour and cannot be bleached at reasonable chemical consumptions. Furthermore, the fibres become coated with a lignin skin and are therefore stiff and rigid, which results in poorer strength and absorption properties. Consequently, it has only been possible to produce chemithermomechanical pulp (CTMP) of high brightness and good absorbency by preheating and refining at a temperature of at most 140.degree. C. High brightness is especially important when producing tissue pulp.
DE-A-27 14 730 describes a process for producing a chemically modified thermomechanical pulp where the wood material is preheated at a temperature of 135.degree.-200.degree. C. during 1-30 minutes. The time used according to the examples is of the order of 10 minutes. To obtain the desired flexibility an energy input of twice the normal is required.